Little refactor and add test cases to conf.py

Little refactor and add test cases to conf.py

- change BrainConfigParser.read() to read_one() since the code only
    use this function to read one config file. It's simpler.
- remove BrainConfigParser.check_opt() since no code use it.
- remove else branch in ConfigMgr.get() since the code can still
    work if val is empty string. Also add a test case for it.
- remove ConfigMgr.set() since no code use it. passwdx set back
    in _check_passwd(), and we will not use this set() function to
    add option into config file.
- change implementation of Fixture class, it doesn't call start()
    and stop() methods which introduced by mock0.7
- add more test cases for conf.py

Change-Id: I598f8943f98f067aa584fa4be811492c70226f23